A Schiff base is an imine condensation product of an aldehyde and a primary amine. Formation of a Schiff base may be illustrated by the following reaction: ##STR1## Where R and/or R' are aliphatic substituents, the C.dbd.N (imine) bond of a Schiff base is known to be very unstable. Typically, the C.dbd.N bond is stabilized by reduction with sodium borohydride or sodium cyanoborohydride, as represented by the following reaction: ##STR2## Stabilization of the C.dbd.N bond may also be achieved through the attachment of an aryl(s) to the imine carbon or nitrogen, or if a hydroxyl or second nitrogen is bonded to the imine nitrogen.
Schiff base linkages have been used for conjugation of glycoproteins in general, and for conjugation of immunoglobulins in particular. In a typical reaction scheme, oligosaccharide moieties present on an immunoglobulin molecule are oxidized to form one or more aldehyde groups. The resultant immunoglobulin aldehyde(s) is reacted with a primary amine to form a Schiff base, which is then stabilized by reduction.
A prototypical Schiff base conjugation procedure (as described above) suffers from numerous disadvantages. First, the immunoglobulin (glycoprotein) molecule is subjected to very harsh oxidizing conditions in order to generate free aldehyde groups. This harsh oxidation may result in impairment of the biological activity of the immunoglobulin molecule. Second, stabilization of the Schiff base conjugate is accomplished through exposure of the conjugate to a harsh reducing agent, which also may adversely affect the biological function of the immunoglobulin moiety. Third, the number of substituents that may be conjugated by Schiff base linkage to immunoglobulin aldehyde groups is limited by the number of carbohydrate moieties present on a particular immunoglobulin molecule. For instance, the amount of carbohydrate present on an immunoglobulin molecule may vary between 2-3% for IgG and 9-12% for IgM, IgD and IgE (I. M. Roitt et al, "Immunology", Gower Medical Publishing Ltd., 1985, p. 5.2).
Alternatively, an aldehyde generated on an immunoglobulin molecule may be reacted with a hydrazide to form a hydrazone, according to the following reaction scheme: ##STR3## Hydrazones are more stable than Schiff bases formed by the reaction of an aldehyde and a primary amine, and thus do not require reduction after formation of the linking bond. However, this reaction scheme suffers from several disadvantages: (1) the immunoglobulin must still be oxidized to generate free aldehyde groups; and (2) the degree of conjugation is limited by the number of carbohydrate moieties present on the immunoglobulin molecule.